The Gangdise zone is widely an important volcanic-magmatic arc belt, which is related with the northward subduction of Yarlung Zangbo oceanic crust and continent-continent collision between India and Eurasia from Late Yanshanian to Himalayan. However, the tectonic attribute of Paleozoic to Mesozoic in Gangdise zone is always a brainteaser. The paper probes into the enigma on the basis of the present data about stratigraphy, paleontology and petrology that obtained from the new-round geological survey(at a scale of 1:250, 000) carried out by the China Geological Survey in south Tibet.
In Himalaya and Gangdise area, the folded basement formed in Pan-Africa period at about 550Ma and the sedimentary cover of O-S is unconformable upon the underlying strata. We consider the Himalaya and Gangdise area not only having the same sedimentary cover in Early Paleozoic but belonging to the platform sedimentation of the passive continental margin on north margin of Gondwanaland also.
In Late Paleozoic, lots of volcanic rocks appeared in Gangdise, including the basic to intermediate volcanic rocks in Nuocuo Formation (C1), basic to acidic volcanic rocks in Laigu Formation(C2), and basic volcanic rocks in Angjie Formation (P1), Laga Formation (P1), and Xiala Formation (P2). All the volcanic rocks are characteristic of the island arc, and are different from those of the extensional setting in Himalayas at the same period. According to the geological and geochemical information, and together with the sedimentary gap between P1 and P2 in Gangdise, we conclude that Gangdise zone transformed into an active continental margin in Late Paleozoic on the background of the passive continental margin in Early Paleozoic, namely, the first island arc orogenesis of Gangdise occurred.
The magmatic activity of T is characteristic of the island arc, including the granite (Rb-Sr age is 227Ma-129Ma) in eastern Gangdise, phenocryst granodiorite (concordant age of zircon is 217.1±3.4Ma) on south hillside of Gangdise, and intermediate-acid volcanic rocks in T2-3 in south of Naqu county and north of Lhasa. On the basis of the magmatism data, and combined with the lack of T1-2 strata, unconformity between T3 or J2-3 and the underlying strata in Gangdise, we consider that Mesozoic Gangdise zone is still island arc on an active continental margin, which is related to the southward subduction of Tethyan Oceanic crust under the Gangdise.